Cracking in homogeneous state



Oct. 31,1933. E, B PECK 1,933,507

CRACKING IN I'IOBIIQG'ENE'OUSv STATE Fiid Aug. 21, 1929 INVENTOR ATTORNEY Patented Oct. 31, 1933 v 1,933,501 omcxme m HOMOGENEOUS STATE Edward B. Peck, Elizabeth, N. J., assignor to Standard Oil Development company, a corporation oi. Delaware Application August 21, 1929. Serial No. 387,339

3 Claims. (01. 196-58) This invention relates to improvements in cracking hydrocarbon oils. and relates to a cracking under conditions precluding the separation of the oil into liquid and vapor phases of different composition during the heating and the soaking periods, that is, during the time the stock is heated up to cracking temperature and kept at cracking temperature.

One object of the invention is to provide a cracking process in which the deposition of coke is greatly diminished.

Another object is to insure an equal flow of the oil in the heating tubes (cracking coil).

A further object is to crack the oil at higher temperatures than have hitherto been used in purely liquid phase cracking processes and to reduce the time and decrease the size of the apparatus necessary for a certain amount of cracking.

A still further object of this invention is to provide a cracking process in which higher conversions to cracked products can be obtained than in purely liquid phase cracking processes.

One aspect of my process provides for the addition of an intermediary stock, such as kerosene, to the heavy cracking stock, to reduce the pressure under which the process has to .be carried out.

Further objects will be apparent and the invention will be fully understood from the following description, read in connection with the accompanying drawing, in which the figure is a vertical diagrammatic section through suitable equipment.

Pure substances have a critical point which is the end of the vapor pressure curve. The critical temperature for a pure substance is the temperature above which no liquid state can exist and corresponds to a definite pressure and volume at the critical temperature. It is a matter of thermodynamic and experimental proof that no such unique point can exist for a mixture but rather a zone with variations increasing as the number of components in the mixture. Such variations are defined by the phase rule.

I have carried out experiments relating to the critical phenomena of various oil products over a period of several years and arrived at certain conclusions which will be readily understood from the following table:

Tamer-Limits of critical data on petroleum (Bel'erredto gas under 1 atm. pressure.)

It is evident from this table that heavier oils show critical phenomena at higher temperatures and lower pressures, and that within the critical zone large volume changes occur for relatively small temperature changes. In other words, the oil is more elastic in the critical zone than in any other state. Another result from these and similar data is that the wider the distillation range the greater is the pressure required for the critical range. Obviously any product will be homogeneous (i. e. it will be present only in one phase) at temperatures below the critical range when the pressure is kept above the critical, or at any pressure when the temperature is above the critical range. For instance, the 45 A. P. I. distillate will be homogeneous at or below 747 F.

at 390 lbs. sq. in. pressure and any pressure i. e. no separation into liquid and vapor phase takes place, the oil is in the gaseous state. The critical range may be recognized by the great elasticity oi. the oil. Separation in this range is not gradual, as for normal liquid state, but occurs abruptly, with varying ratios of liquid and vapor, depending on the temperature and pressure. The critical zone is unique in that retrograde condensation may occur. If a gas is compressed at constant temperature within the critical range, normal condensation of liquid will occur with increase in liquid to vapor ratio. On increasing pressure this condition continues to a maximum formation of liquid, after which increase in pressure causes liquid to vaporize very rapidly.

Gas oil or other heavy stocks used as raw materials for cracking have their critical zone at high temperatures and comparatively low pressures. For instance, the critical zone of a 34 A. P. I. Mid Continent gas oil is well over 900 F. with pressures of the order of 250300 lbs. sq. in. However, from 600 F. on, cracking takes place which becomes rapid at temperatures ranging above 800 F. and consequently light prodnets are formed so that the pressure required for the critical state increases rapidly while the temperature range of the critical state decreases. After 10 minutes cracking at 850 F. the above gas oil will show critical phenomena at 750 lbs. sq. in. and I have found critical phenomena for highly cracked gas oil at a temperature of 780 F. and a pressure of 1750 lbs. sq. in.

I have discovered that the coke formation in cracking is greatly reduced and a smooth operation becomes possible when the cracking is carried out in sucha manner that the oil is constantly in a homogeneous state during the periods of heating and soaking (cracking). For the sake of brevity we shall call such a process "cracking in the homogeneous state. A possible explanation of this circumstance during the cracking period is predicated on the fact that coke is a byproduct of polymerizing reactions. Such reactions being multimolecular are suppressed by dilution which is at a maximum when all the constituents are mutually dissolved. While multimolecular reactions are suppressed by dilution they are also accelerated by increase in pressure. If these factors were the only ones operating the coke reactions would be at a minimum at the lowest pressure for homogeneous state. I have found that coke formation is suppressed as the pressure is increased beyond that for homogeneous state. This is associated with a decided decrease in oleflnes in the light products and I have therefore concluded that coke formation is suppressed by the reaction of light oleflnes with the heavy polymerizable products. Again, if the oil separates into a vapor and liquid during the heating period, there will be an unequal flow, some of the liquid oil lagg g behind and becoming burned (over-cracked) on the hot surface of the heating tubes.

There are two methods available for cracking in the homogeneous phase. First; cracking below the critical temperature range and increas-- ing the pressure to such extent that the oil does not vaporize at all. In this case the oil remains liquid during the heating and the soaking periods, since the oil at the lower stages of conversion has lower critical pressure and higher critical temperature range. This method has the disadvantage that both the cracking temperature and the conversion to which the cracking proceeds must be kept comparatively low. Further, the low cracking temperature entails a considerable soaking time since the rate of cracking is very slow at low temperatures. The second method consists of cracking under such conditions of temperature and pressure that the oil is above the critical range or in the gaseous state in the final stage of cracking, that is, when the conversion required is attained. In the last mentioned case the oil must have passed from the liquid state into the gaseous one at some stage of the heating or the soaking period. Such transition is possible without evaporation but by uniform expansion from the cold liquid to the hot gaseous state. In this case the transition is homogeneous, i. e., the liquid oil will go over into the gaseous state without separating into a liquid and a vapor phase during the transition, and the application of this characterizes my invention. I have discovered that by proper regulation of the composition of the oil, temperature, pressure, and the heating rate, the cracking can be made homogeneous. For instance, if a 34 A. P. I. Mid Conti nent gas oil is cracked at 850 F. to a conversion of 35% by volume of gasoline under a pressure of 900 lbs. sq. in. it will be in the critical stage at 35% conversion to gasoline. Passing this conversion, two diiferent phases, liquid and vapor, appear in the cracking vessel. At all lower conversions the oil is homogeneous and liquid, so that in this case the cracking will be homogeneous independently of the heating rate at which the oil was brought up to the cracking temperature, as long as the conversion to gasoline is kept below 35%.

Cracking the same gas oil at 850 F, and under 1000 lbs. sq. in. pressure, the oil will be inthe critical stage at 38% conversion to gasoline and the cracking will be homogeneous at lower conversions, say 37%, independently of the time in which the oil was brought up to cracking temperature. The conditions are analogous in cracking under 750 lbs. sq. in. pressure (temperature 850 F.) in which case separation of liquid and vapor phase takes place at 25% conversion. At 850 F. and 350 lbs. sq. in. pressure, it is impossible to carry out the cracking in the homogeneous state and separation in liquid and vapor phase occurs at less than 1% conversion.

At higher temperatures it becomes easier to carry out the cracking in the homogeneous state. For instance, at 950 F. under 750 lbs. sq. in. pressure it is possible to carry the cracking far beyond the degree corresponding to the most strenuous cracking given above as example for the .operation at 850 F., without observing a separation into liquid and vapor. At the beginning of the heating period the oil is in the liquid state and at the end of the cracking period it is in the gaseous state; the transition occurred through the critical stage in a continuous manner. In such cases the rate of heating becomes an important factor. Cracking will naturally take place during the time while the oil 'is heated up to 950 F.

. 'If the heating up is eifected in a short time so that the cracking is below the conversion of 25 gasoline when the temperature of 850 F. is reached, the cracking will be homogeneous. However, if the heating is carried out at a slow rate and the oil is allowed to crack to 25% gasoline conversion at a time when the temperature reached is 850 F. or lower, separation into liquid and vapor phase takes place suddenly when the conditions of composition temperature and pressure are satisfied. This separation may vary from. almost all liquid to almost all vapor within slight changes of any of the variables. In this case the cracking has not been carried out in tical conversion limit, i. e.'40% gasoline in the homogeneous state under 900 lbs. sq. in. pressure (or higher) if the rate of heating the oil to that temperature is not exceptionally slow. The conditions are about the same for other gas oils. At higher temperatures the pressure required for the maintenance of homogeneous state in cracking is lower, as illustrated in the general, I have observed that middle fractions,

ranging between a heavy cracking stock and the gaseous and other. light products resulting from the cracking, have a marked solvent effect on the heavy and light ends and consequently lower the pressure necessary for homogeneous phase cracking After the desired degree of cracking is obtained, the various products are separated by any suitable method known to the art. The separation does not form part of my invention and therefore it will be only briefly discussed. Various equipment known in the art maybe used. I may use, for instance, the apparatus and the method of separation described in U. S. patent application Serial No. 178,547 of John S. Harrison.

Referring to the drawing, the apparatus and the process will be jointly described.

The feed is charged by means of the high pressure pump 25 through line 26 to the cracking coil, consisting of a series of tubes 1 arranged in a furnace 2, and thence through line 3 into the soaking drum 4. The cracked products leave the latter by line 5 and after passing through the tar separator 6 they are discharged throughline 10 and spray head 11 into the accumulator 12.

- Release valves '7 and 10' allow the maintenance of the desired pressure on the soaking drum and on the tar separator independently. The separated tar' leaves by line 8, is cooled in 9 and stored in a suitable tank. Fresh feed is charged into the accumulator through line 13. Any desired fraction of the fresh feed may be passed through coils 14 whereby a certain amount of preheat is obtained. The heavy uncracked hydrocarbons condense in the accumulator and are returned together with the fresh feed to cracking coil 1 while the distillate leaves through line 1'7 and is fractionated in the bubble tower 15 in a manner well known in the art. The bubble tower contains a number of plates 18, provided with bell caps and reflux lines. The gas and gasoline leave tower 15 by line 19, the gasoline is condensed in condenser 20 and separated from the gas in tank 21. The condensate in the bubble tower may either be removed from the system or returned to the coil or to the accumulator by lines 22, 23, 24 respectively. Valved line 27 serves for adding kerosene or other middle fracliquid and vapor phases. be regulated so as to imitate the corresponding tion to thecharging stock as explained above. The accumulator and the bubble tower are preferably held under a pressure of about 60 lbs. sq. in. The pressure and the temperature of the soaking drum are chosen according to the composition of the charging stock -in line 26 and the conversion in the soaking drum in such a manner that under the rate of heat pick-up in coil 1 the stock remains constantly in the homogenous state, both in coil 1 and in soaking drum 4. Although this temperature and pressure'can be calculated with good accuracy on the basis of theories relating to cracking and heat transfer, I usually prefer to determine them by means of experiments. For this purpose I take a sample of the charging stock (which, of course, will diiferin composition from the fresh feed) and subject it to heating in a thick-walled glass tube heated by electrical means. The stock in the-tube is held under the hydrostatic pressure of a'mercifry column and it can be ascertained by visual observation whether the stock is in the homogeneous phase or separated into The rate of heating can rate in cracking will and the sample is held under the specified'condition's of pressure and temperature as long as the corresponding chargeis held in'the soaking drum when operating on the large scale.-

In practice, where the pressure and feed rate are largely predetermined, it is convenient to control the conditions for homogeneous cracking by the amount and composition of the cycle stock returned for cracking. It is not generally desirable to obtain maximum conversion of the oil in a single heating for two reasons; (1) the pressure for homogeneous cracking increases rapidly with every percent increase in conversion and (2) the gasoline cracks as well as the stock until a point is reached where the gasoline cracks as fast as formed and finally faster. To overcome these two disadvantages I choose to crack to a predetermined conversion and-then separate the gasoline and gas, middle fraction and tar by fractionation according to known methods. The middle fraction is returned for further cracking. This middle fraction is more refractory than fresh gas oil. When it is mixed with the fresh feed it has two effects 1) reduces the rate of cracking 125 which reduces the pressure required for homogeneous cracking and (2) it increases the solubility of light and heavy constituents from cracking again with the result that lower pressure is required for homogeneous state. 130

The following examples will illustrate the operation of my process and the yields obtained:

A feed stock of 31 A. P. I. consisting of 50% cycle stock and 50% fresh gas oil is fed at a pressure of 900 lbs. per sq. in. on the soaking drum of 6'x40 and at a rate of- 12,800 gals/hr. This oil leaves the heating coils at 885 F. with a gasoline conversion of 8.12% and leaves the soaking drum at 850 F. and a conversion of 28%, 400 F. end point gasoline on the totalfeed, corresponding to 56% gasoline yield on the fresh gas oil. The gas yield was 5.8% but only traces of coke were formed. What coke was formed was that due to starting up the unit and attaining steady state, for the conversion remained uniform thereafter showing no clogging of the unit.

If the same unit is run at the same rate but half of the cycle stock is drawn off for furnace oil, the conversion is not steady and coke forms 150 in the'tubes and reaction chamber because the homogeneous state is not realized.

The following example is one of cracking in the homogeneous state under very high pressure and without recycling. A 31.7 A. P. I. Midcontinent gas oil with a feed rate of 13,070 gal. per hour and a pump pressure of 2,460 pounds and an outlet pressure of 2000 pounds leaves the coil at 850 and 13% conversion to 400 E. P. gasoline and leaves the reaction drum as homogeneous gas at 840 F. and 39% conversion to 400 E. P. gasoline, 6.5% gas and 54.5% middle fraction and tar.

When operating at higher temperatures the rate of cracking increases fast and correspondingly the time necessary to obtain a certain degree of decomposition decreases. In such a case the soaking drum can be made considerably smaller or entirely dispensed with and line 3 connected directly to the tar separator 6; the whole cracking being done in cracking coil 1. In order to insure the separation of the tar, cold feed, such as crude oil, gas oil, etc. may be introduced into the hot cracked product just before it is discharged into the tar separator. The cooling thus obtained will precipitate out the tar which must be removed from the system if excessive coke formation is to be avoided.

I am aware that, since early dates in the art of the cracking, it has been proposed to crack in the liquid phase, that is, to hold such a high pressure in conjunction with a low temperature of cracking that the charging stock is in the liquid state during the heating and soaking periods. I therefore do not claim such a process. But it has not been recognized until the present invention that a petroleum oil can be heated and cracked under such conditions that the oil 'is above the critical range or in the gaseous state at the end of the cracking period and yet at no time during the heating and soaking periods does it separate into a liquid and a vapor phase. This, and the methods adopted to realize this condition, are the essential features of the present invention.

Some modifications of my process have been discussed above. Various other modifications can be made without departing from the spirit of this invention. Furthermore, I do not wish to be limited by any theory or specific data given in way of illustration, but only by the appended claims in which I intend to claim all inherent novelty of my process.

I claim:

1. The process of cracking which comprises passing liquid hydrocarbon oil in a continuous stream through a heating zone of narrow cross section into an enlarged cracking zone, maintaining the oil at cracking temperature in the cracking zone for a time sufiicient to obtain the desired conversion into cracked products, regulating the cracking temperature and the pressure in the cracking zone so that the oil therein is in a homogeneous gaseous state, maintaining a pressure in the heating zone equal to that in the cracking zone plus the frictional pressure drop due to the passage of the oil through the heating zone, maintaining the pressure in the cracking zone and the rate of heating in the heating zone sufiiciently high to insure a homogeneous state of the oil at any cross section in both the heating and the cracking zones, and-separating the cracked oil into desirable fractions.

2. The process according to claim 1, in which the hydrocarbon oil to be cracked consists of gas oil or other heavy oil, and a lighter oil is added to the oil before the heating zone to reduce the pressure to be maintained in the heating and cracking zones.

3. The process according to claim 1, in which the hydrocarbon oil to be cracked consists of gas oil or other heavy oil; and kerosene or heavy naphtha is added to the oil before the heating zone to reduce the pressure to be maintained in the heating and cracking zones.

EDWARD B. PECK. 

